It is often very important to know what the permeabilities of soil are at a particular site, at various depths, in the absence of a water table. This information is useful, for example, in predicting the long-term irrigation suitability of farmlands. The primary interest here is in assessing the potential for detrimental water table formation. Once the soil's permeability is known, from such measurement of percolation rates at a site, it becomes possible to estimate the subsurface outflow capability for excess irrigation water (deep percolation) and, therefore, to predict the rate of water table buildup, if any. Internal drainage requirements, if needed, can also be assessed using this information.
Likewise, the ability of the ground to absorb and transmit water through the soil, as distinct from surface water run-off, is a very important factor to be considered in selecting the site of a building, factory, dam or any heavy stationary structure whose physical integrity and stability depend on the ability of the underlying soil to conduct away rainfall at a satisfactory rate. Determination of the ability of soil at a given construction site to so conduct away incident moisture provides assurance to an architect that there will be adequate support for the prospective structure. Such data may significantly affect both the design and its cost, as well as the willingness of local authorities to approve its construction.
A common practice is to drill or auger a circular hole a few inches in diameter to the depth of a few feet, then to pour water into the hole to an observed level and, thereafter, to observe the rate at which the water level drops in the test hole. Two recently issued patents on appartus for such purposes are U.S. Pat. No. 3,945,247 titled "Percolation Gauge," issued to Anderson on Mar. 23, 1976, and U.S. Pat. No. 4,182,157 titled "Soil Percolation Testing Apparatus" issued to Fink on Jan. 8, 1980. The Anderson apparatus has a hooked rod whose end is periodically moved to the water level in a test hole at recorded time intervals, thereby to provide data on the soil percolation rate. The Fink apparatus has a guided vertical rod supported by a float which drops in the test hole previously filled with water to hold up the float. In both these inventions, the user must determine both the fall of water level in a test hole as well as elapsed time, and then calculate the average percolation rate over that time through the soil at the test location. Neither invention teaches how to determine, directly, the rate at which water percolates through soil when the water level at a test site is held constant, as would be the case where there is prolonged rainfall or where a dam or nearby flooding requires the soil at the test site to conduct away percolating water over extensive periods.
There is, therefore, a need for apparatus and a method which provide a user with direct information on the rate at which water percolates through soil at a test location.